The gate threshold voltage V.sub.th of an insulated gate semiconductor device is given by ##EQU1##
In this equation .phi..sub.MS is the difference in work functions between the gate electrode and the substrate, .phi..sub.F is the difference in Fermi potentials of the two materials, Q.sub.B is a quantity determined by the impurity concentration in the substrate, Q.sub.SS is the electric charge in the gate insulating film and C.sub.O is the electrostatic capacitance between the substrate and the gate electrode.
In this formula, .phi..sub.MS, .phi..sub.F and Q.sub.B /C.sub.O can be controlled comparatively readily by proper design and production technique. However Q.sub.SS /C.sub.O is not only difficult to control but is subject to change subsequent to production, as the result of which integrated circuits of the type described have not been as reliable as would be desired.
The field insulating film which overlies the gate and the substrate is formed by chemical vapor deposition as the result of which it is difficult to decrease Q.sub.SS. Further, where the registry of the gate and the demarcation between the conductive portion of the substrate and the stopper is not perfect, the threshold voltage of the portion facing the field insulating film can become extremely small or even negative. Consequently, unintended current can flow between the source and drain of a device.